c++ lambdas how to capture variadic parameter pack from the upper scope

Question

I study the generic lambdas, and slightly modified the example, so my lambda should capture the upper lambda's variadic parameter pack. So basically what is given to upper lambda as (auto&&...) - should be somehow captured in [=] block.

(The perfect forwarding is another question, I'm curious is it possible here at all?)

#include <iostream>
#include<type_traits>
#include<utility>


// base case
void doPrint(std::ostream& out) {}

template <typename T, typename... Args>
void doPrint(std::ostream& out, T && t, Args && ... args)
{
    out << t << " ";                // add comma here, see below
    doPrint(out, std::forward<Args&&>(args)...);
}

int main()
{
    // generic lambda, operator() is a template with one parameter
    auto vglambda = [](auto printer) {
        return [=](auto&&... ts) // generic lambda, ts is a parameter pack
        {
            printer(std::forward<decltype(ts)>(ts)...);
            return [=] {  // HOW TO capture the variadic ts to be accessible HERE ↓
                printer(std::forward<decltype(ts)>(ts)...); // ERROR: no matchin function call to forward
            }; // nullary lambda (takes no parameters)
        };
    };
    auto p = vglambda([](auto&&...vars) {
        doPrint(std::cout, std::forward<decltype(vars)>(vars)...);
    });
    auto q = p(1, 'a', 3.14,5); // outputs 1a3.14

    //q(); //use the returned lambda "printer"

}

Answer 1

Perfect capture in C++20

template <typename ... Args>
auto f(Args&& ... args){
    return [... args = std::forward<Args>(args)]{
        // use args
    };
}

---

C++17 and C++14 workaround

In C++17 we can use a workaround with tuples:

template <typename ... Args>
auto f(Args&& ... args){
    return [args = std::make_tuple(std::forward<Args>(args) ...)]()mutable{
        return std::apply([](auto&& ... args){
            // use args
        }, std::move(args));
    };
}

Unfortunately std::apply is C++17, in C++14 you can implement it yourself or do something similar with boost::hana:

namespace hana = boost::hana;

template <typename ... Args>
auto f(Args&& ... args){
    return [args = hana::make_tuple(std::forward<Args>(args) ...)]()mutable{
        return hana::unpack(std::move(args), [](auto&& ... args){
            // use args
        });
    };
}

It might be usefull to simplify the workaround by a function capture_call:

#include <tuple>

// Capture args and add them as additional arguments
template <typename Lambda, typename ... Args>
auto capture_call(Lambda&& lambda, Args&& ... args){
    return [
        lambda = std::forward<Lambda>(lambda),
        capture_args = std::make_tuple(std::forward<Args>(args) ...)
    ](auto&& ... original_args)mutable{
        return std::apply([&lambda](auto&& ... args){
            lambda(std::forward<decltype(args)>(args) ...);
        }, std::tuple_cat(
            std::forward_as_tuple(original_args ...),
            std::apply([](auto&& ... args){
                return std::forward_as_tuple< Args ... >(
                    std::move(args) ...);
            }, std::move(capture_args))
        ));
    };
}

Use it like this:

#include <iostream>

// returns a callable object without parameters
template <typename ... Args>
auto f1(Args&& ... args){
    return capture_call([](auto&& ... args){
        // args are perfect captured here
        // print captured args via C++17 fold expression
        (std::cout << ... << args) << '
';
    }, std::forward<Args>(args) ...);
}

// returns a callable object with two int parameters
template <typename ... Args>
auto f2(Args&& ... args){
    return capture_call([](int param1, int param2, auto&& ... args){
        // args are perfect captured here
        std::cout << param1 << param2;
        (std::cout << ... << args) << '
';
    }, std::forward<Args>(args) ...);
}

int main(){
    f1(1, 2, 3)();     // Call lambda without arguments
    f2(3, 4, 5)(1, 2); // Call lambda with 2 int arguments
}

---

Here is a C++14 implementation of capture_call:

#include <tuple>

// Implementation detail of a simplified std::apply from C++17
template < typename F, typename Tuple, std::size_t ... I >
constexpr decltype(auto)
apply_impl(F&& f, Tuple&& t, std::index_sequence< I ... >){
    return static_cast< F&& >(f)(std::get< I >(static_cast< Tuple&& >(t)) ...);
}

// Implementation of a simplified std::apply from C++17
template < typename F, typename Tuple >
constexpr decltype(auto) apply(F&& f, Tuple&& t){
    return apply_impl(
        static_cast< F&& >(f), static_cast< Tuple&& >(t),
        std::make_index_sequence< std::tuple_size<
            std::remove_reference_t< Tuple > >::value >{});
}

// Capture args and add them as additional arguments
template <typename Lambda, typename ... Args>
auto capture_call(Lambda&& lambda, Args&& ... args){
    return [
        lambda = std::forward<Lambda>(lambda),
        capture_args = std::make_tuple(std::forward<Args>(args) ...)
    ](auto&& ... original_args)mutable{
        return ::apply([&lambda](auto&& ... args){
            lambda(std::forward<decltype(args)>(args) ...);
        }, std::tuple_cat(
            std::forward_as_tuple(original_args ...),
            ::apply([](auto&& ... args){
                return std::forward_as_tuple< Args ... >(
                    std::move(args) ...);
            }, std::move(capture_args))
        ));
    };
}

---

capture_call captures variables by value. The perfect means that the move constructor is used if possible. Here is a C++17 code example for better understanding:

#include <tuple>
#include <iostream>
#include <boost/type_index.hpp>


// Capture args and add them as additional arguments
template <typename Lambda, typename ... Args>
auto capture_call(Lambda&& lambda, Args&& ... args){
    return [
        lambda = std::forward<Lambda>(lambda),
        capture_args = std::make_tuple(std::forward<Args>(args) ...)
    ](auto&& ... original_args)mutable{
        return std::apply([&lambda](auto&& ... args){
            lambda(std::forward<decltype(args)>(args) ...);
        }, std::tuple_cat(
            std::forward_as_tuple(original_args ...),
            std::apply([](auto&& ... args){
                return std::forward_as_tuple< Args ... >(
                    std::move(args) ...);
            }, std::move(capture_args))
        ));
    };
}

struct A{
    A(){
        std::cout << "  A::A()
";
    }

    A(A const&){
        std::cout << "  A::A(A const&)
";
    }

    A(A&&){
        std::cout << "  A::A(A&&)
";
    }

    ~A(){
        std::cout << "  A::~A()
";
    }
};

int main(){
    using boost::typeindex::type_id_with_cvr;

    A a;
    std::cout << "create object end

";

    [b = a]{
        std::cout << "  type of the capture value: "
          << type_id_with_cvr<decltype(b)>().pretty_name()
          << "
";
    }();
    std::cout << "value capture end

";

    [&b = a]{
        std::cout << "  type of the capture value: "
          << type_id_with_cvr<decltype(b)>().pretty_name()
          << "
";
    }();
    std::cout << "reference capture end

";

    [b = std::move(a)]{
        std::cout << "  type of the capture value: "
          << type_id_with_cvr<decltype(b)>().pretty_name()
          << "
";
    }();
    std::cout << "perfect capture end

";

    [b = std::move(a)]()mutable{
        std::cout << "  type of the capture value: "
          << type_id_with_cvr<decltype(b)>().pretty_name()
          << "
";
    }();
    std::cout << "perfect capture mutable lambda end

";

    capture_call([](auto&& b){
        std::cout << "  type of the capture value: "
          << type_id_with_cvr<decltype(b)>().pretty_name()
          << "
";
    }, std::move(a))();
    std::cout << "capture_call perfect capture end

";
}

Output:

  A::A()
create object end

  A::A(A const&)
  type of the capture value: A const
  A::~A()
value capture end

  type of the capture value: A&
reference capture end

  A::A(A&&)
  type of the capture value: A const
  A::~A()
perfect capture end

  A::A(A&&)
  type of the capture value: A
  A::~A()
perfect capture mutable lambda end

  A::A(A&&)
  type of the capture value: A&&
  A::~A()
capture_call perfect capture end

  A::~A()

The type of the capture value contains && in the capture_call version because we have to access the value in the internal tuple via reference, while a language supported capture supports direct access to the value.